Power semiconductors have long been used in large commercial applications such as ground based power supplies, elevators, electric locomotives, etc. In these applications power semiconductor package size and weight, as well as the type and amount of cooling required was not of great concern. With the increased use of these devices in aerospace applications, however, package size and the associated cooling requirements become very important.
On an aircraft every pound of additional weight results directly in increased fuel burn, and in turn higher operating costs for the airlines. Also, every additional cubic foot used by required operating equipment detracts from the amount of revenue generating cargo which the airplane can carry. With the increased use of power semiconductors on aircraft, and in particular as key elements of total aircraft power systems, the effort to minimize package size and reduce weight is paramount. Countering this effort, however, is the requirement for more and more power usage on the modem aircraft. The substantial financial penalties levied on the suppliers of these power systems for equipment which is overweight and oversize will guarantee that the effort for small package size and reduced weight will continue.
One problem associated with increased power requirements in a reduced package size is that an increased amount of heat, as generated by the non-perfect nature of the devices, must be removed from a smaller area. The concept of thermal resistance, see equation 1, is a measure of the difficulty of heat transfer from one place to another. EQU Thermal Resistance=R.sub.TH =L/k(A) (1)
where L=32 material thickness PA1 k=thermal conductivity PA1 A=surface area
From this equation one can see that if all else remains constant, a decrease in surface area results in an increased thermal resistance, that is, it becomes more difficult to remove the increased amount of heat generated in the smaller package. This is a problem for power semiconductors as many exhibit a phenomenon known as thermal runaway. As the temperature of the device rises, the resistance to electrical current flow drops which results in increased current flow and increased heat generation. Increased heat generation results in decreased resistance to electrical current flow which results in increased heat generation, and so forth. This situation continues until the device self destructs.
One way to remove the heat generated by these power semiconductors is to increase the cooling flow by use of fans or air conditioners, or even by use of fluid cooling systems. The problem with these systems for many applications is increased cost and weight. A more practical solution is to attach a heat sink to the power semiconductor package and allow the heat generated to be transferred to the fins of the heat sink. In this way, the area from which heat may be removed is increased. Referring back to the concept of thermal resistance, see equation 1, an increase in area will result in a decrease in thermal resistance, that is, it becomes easier to remove the heat.
With the increasing amounts of power that the aerospace power semiconductors are required to handle, a small loss in efficiency can have disasters effects. This statement holds true for the efficiency of heat removal as well. Because the power semiconductor package surface and the heat sink surface are not polished, perfectly smooth surfaces, and because they are bolted together to lower the construction and maintenance costs, the area of actual contact between these two is small. In a dry electronic compartment, these gaps are filled with air and, although these air gaps are small, the resulting thermal resistance is quite large, i.e. it is difficult to transfer heat through these air gaps.
The difficulty or ease of heat transfer through a material is inherent within the material itself, and is denoted as a number known as the material's thermal conductivity. Copper, for example, is a very good thermal conductor and has a thermal conductivity of 384. Air, on the other hand, is a very poor thermal conductor with a thermal conductivity of 0.015 to 0.03, over 12,000 times smaller than that of copper.
Since a perfectly smooth surface is difficult to obtain and a perfect contact between the power semiconductor and the heat sink is even more difficult to obtain, it is known within the art to fill the small air gaps resulting from the surface irregularities with a better thermal conductor than air. Although some of the air gaps can be closed with increased surface pressure, material limitations will not allow this method to be greatly effective. Within the art, the use of a silicone grease or an indium foil has become the preferred method of filling the air gaps of the surface interface between the power semiconductor and the heat sink. These materials have been chosen because of special properties unique to them.
Although silicone grease has a thermal conductivity of only approximately 10 times that of air, it is often used as an interface media because it tends to fill only the air gaps and is displaced by, that is, it does not interfere with, the power semiconductor/heat sink surface contact points. This results in a much more efficient transfer of heat to the fins of the heat sink. As with any fluid material, however, the problem of migration is apparent with the use of a silicone grease. This results in a loss of material where it is supposed to be and an increase of material where it is not supposed to be. This loss of the silicone grease from the surface air gaps, in addition to containing other surfaces, results in a loss of heat transfer efficiency as air is once again the transfer media.
One prior art method used to increase the heat transfer efficiency while avoiding the problems associated with a silicone grease is to use an indium foil as a heat transfer interface media. Although indium's thermal conductivity is 2,000 to 4,000 times better than air and approximately 300 times better than that of silicone grease, its effectiveness is limited by its minimum handling thickness, which is on the order of 0.002 inch, and as can be seen from equation 1, the larger the thickness, the greater the thermal resistance. The net result of the thicker foil on overall power semiconductor to ambient thermal resistance is about the same as if a silicone grease were used as the thermal transfer interface media. Indium foil fills the surface irregularities by cold flowing and cold welding itself to the entire surface of both the power semiconductor and the heat sink, thus eliminating any air gaps which otherwise would have formed. The problem with indium, besides the high cost of the material itself, is that it increases the cost of repair. Increased repair costs are associated with the cold welding nature of indium. When the power semiconductor is separated from the heat sink, the indium foil, which has cold welded itself to both surfaces, tears, forming 0.002 inch thick mesas on the power semiconductor and 0.002 inch thick mesas on the heat sink. Because the thickness needs to be limited to keep the thermal resistance to a minimum, the foil must be scraped from both surfaces before new foil can be applied and the two re-attached.
The present invention is directed to overcoming one or more of the above problems by using a material never before considered appropriate in the art for the intended application.
The xylylene polymer known as parylene is used within the art as a conformal coating of circuit boards, hybrid circuits, and semiconductors, in addition to use as an electrical insulator for small electrical components such as miniature stepping motors used in wristwatches. Parylene is utilized in these applications because of its dielectric strength, that is, its ability to resist the transmission of electrical energy, its excellent chemical resistance and its resistance to fungal attack. Parylene, though, is not a good thermal conductor, as its thermal conductivity ranging from 0.082 to 0.12, only three (3) to eight (8) times better than air itself.